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<head>
  <meta charset="UTF-8">
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  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
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<body>
  <script>
    // var num = 5

    // var result = ++num + num++  // 6+ 6

    // console.log(result, num)  // 12 7

    var num = 5

    var result = num-- + --num  // 5 + 3
    console.log(result)  //  8
  </script>
</body>
</html>